1. Field of the Invention
The invention is directed to circuits that provide programming high voltages, and more particularly, to circuits that program non-volatile type memory cells including floating-gate transistors as storage elements. The invention can be used further for programming other types of memory cells, including static and dynamic memories, insofar as the progress of their programming voltage needs to be checked. With the invention, it is possible to control the development of this programming voltage when it is set up. More generally, the invention provides control for the progress of any voltage whatsoever.
2. Discussion of the Related Art
In the field of non-volatile memories, the cells of which are EPROM, EEPROM or Flash EPROM type cells, it is usually necessary to apply a high voltage of about 20 volts to store charge in a floating gate of a floating-gate transistor. Normally, an external supply circuit possesses a supply circuit to produce this high voltage. However, when the integrated circuit for which a programming high voltage has to be produced is a mounted circuit, in particular when it is a chip of a chip card or a circuit board, the programming high voltage has to be produced within the integrated circuit. It is possible, using a general supply low voltage, for example equal to 5 volts, to produce high internal voltages, for example of the order of 20 volts.
The development of technologies is now leading to the recommending of general supply voltages of lower values, for example 3 volts, or even 1.8 volts. The usefulness of these approaches using very low voltage is that the total energy dissipated in an integrated circuit is reduced: its temperature is therefore lower, and it works more efficiently. Furthermore, with the miniaturization of circuits, breakdown voltages or voltages corresponding to a change in state are reduced so that a low voltage supply becomes a necessity. However, despite this miniaturization, it is still necessary to have recourse to programming high voltages.
The principle of the production of a programming high voltage within an integrated circuit consists of the use of a voltage pull-up circuit within this integrated circuit. A voltage pull-up circuit is constituted for example by a Schenkel type multiplier or else by a charge pump. The technique of making these pull-up circuits is such that, for a given supply voltage, for example 5 volts, the value of the high voltage produced is limited by a maximum, for example 20 volts. If the supply voltage is lower, for example 3 volts, it is still possible to produce potential differences of 18 volts by producing positive voltages and negative voltages in the circuit. However, there then arises a problem of cost related to this complexity of the circuit. Such costs and complexity are increased if the supply voltage is lower than 3 volts.
Besides, fan-out of the voltage pull-up circuit, especially when it is a charge pump, decreases as the programming high voltage produced increases above the starting general supply voltage. In view of the fact that memory arrays, namely bit lines, do not have perfect insulation, there are electrical leakages which, in combination with technical constraints related to making the voltage pull-up circuit, lead to choosing the maximum permissible values for the programming voltages. In practice, it is observed at the present time that the highest possible programming voltage is limited by about 18 volts for a voltage of 1.8 volts.
When a programming high voltage is to be applied to a memory cell, it should not be applied suddenly. Otherwise there is a risk of causing the deterioration of a gate oxide layer placed between a floating gate and a drain or source region of the floating-gate transistor to be programmed. To avoid this problem, a voltage ramp is produced from the programming high voltage. The slope of this ramp, which increases as a function of time, is calibrated so that it remains smaller than a critical slope. The circuit that sets up the voltage ramp has an N type transistor connected by its drain to a source of programming high voltage (that is constant), and by its source to the place of application of the voltage ramp. To the gate of this N type transistor, there is applied a signal that increases regularly with time so as to control the voltage available at the source. It can be seen then that this voltage available at the source follows, except for a drop in voltage VTN, the value of the signal applied to the control gate of this N type transistor.
Now, the drop in voltage really caused in the N type transistor is actually an addition of the following two values: a drop in voltage intrinsic to the N type transistor and a drop in voltage related to the difference between the source voltage of the N type transistor and the substrate voltage of the integrated circuit. Generally, a total difference of 2 volts is the value chosen. This means that all that is available thereafter is a maximum end-of-ramp voltage of 16 volts. If a voltage greater than 16 volts is desired, the DC programming high voltage must be increased to the detriment of the fan-out of the charge pump. Consequently, the problem is not truly resolved for, when the circuit lets through current, this voltage drops.